Phoma stem canker could appear one to two weeks earlier than normal this autumn and growers must be prepared to treat crops accordingly, say experts.
Wet weather throughout the summer means there is plenty of soil moisture to encourage early spore release, Rothamsted’s oilseed rape specialist, Jon West, explains. “We’re probably looking at mid-September release in the southeast, whereas last year phoma was only just starting to come in by the end of September/beginning of October.
“Even if we don’t get much rain from now on, we’re at the time of year when we get heavy dew most nights, which will be enough to aid spore release.”
While many early-sown crops have emerged and are at risk from infection, Dr West says there is no need to panic, as it may be worthwhile delaying treatment slightly anyway. “Research has shown the most severe canker comes from leaves 6-8. It might not be so crucial to protect the really early leaves, as these tend to drop off fairly quickly as the canopy grows, before infection has spread to the stem.”
But The Arable Group’s David Parish says growers should start protecting crops from when the disease is first seen. “Don’t get too hung up on growth stage. When 10-20% plants are affected is largely regarded as the start of when phoma really gets going.”
Product choice comes down to Plover (difenoconazole), Punch C (carbendazim + flusilazole) and Proline (prothioconazole), he suggests. “All three are fairly similar in terms of efficacy, but Proline will be at a premium to the other two.
“Whatever rate you use, you’re only likely to get 3-4 weeks protection,” he continues. “So if treatments are starting in September, products will be running out by October. You can’t expect one spray to last through the whole season.”
Mr Parish says it is also worth destroying any volunteer rape, which could provide a “green bridge” for phoma to spread into this season’s crops.